Could we use llvm-as instead of clang as assembler for llvm by default?

Summary

GHC doesn't currently check if clang is available, only for opt when using -fllvm.

However since we started using clang as assembler both are required.

Compile anything with -fllvm with clang not available but llvm available.

I know this was discussed in the past. But I wondered what caused us to use clang over llvm-as in the first place.

Beyond that if we can't change it we should probably at least check for clang so that we can give a sensible error message when it's not available.

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